mg-lower: route summarization support

[rcgoodfellow]

I noticed on a deployment if we have a pile of routes through the same nexthop that have overlapping prefixes, they all get installed onto the ASIC which is unnecessary.

For example

root@oxz_switch0:~# swadm route ls
Subnet                   Port    Link  Gateway
0.0.0.0/0                qsfp0   0     198.51.100.3
198.51.100.0/24          qsfp0   0     198.51.100.3
198.51.100.1/32          qsfp0   0     198.51.100.3
198.51.100.2/32          qsfp0   0     198.51.100.3
198.51.100.3/32          qsfp0   0     198.51.100.3
172.20.0.0/24            qsfp0   0     198.51.100.3
172.20.1.0/24            qsfp0   0     198.51.100.3
172.20.2.0/24            qsfp0   0     198.51.100.3
172.20.3.0/24            qsfp0   0     198.51.100.3
172.20.4.0/24            qsfp0   0     198.51.100.3
172.20.5.0/24            qsfp0   0     198.51.100.3
172.20.6.0/23            qsfp0   0     198.51.100.3
172.20.11.0/24           qsfp0   0     198.51.100.3
172.20.12.0/24           qsfp0   0     198.51.100.3
172.20.15.16/29          qsfp0   0     198.51.100.3
172.20.15.24/29          qsfp0   0     198.51.100.3
172.20.15.32/29          qsfp0   0     198.51.100.3
172.20.15.100/32         qsfp0   0     198.51.100.3
172.20.15.102/32         qsfp0   0     198.51.100.3
172.20.15.104/32         qsfp0   0     198.51.100.3
172.20.15.105/32         qsfp0   0     198.51.100.3
172.20.15.106/32         qsfp0   0     198.51.100.3
172.20.15.107/32         qsfp0   0     198.51.100.3
172.20.15.111/32         qsfp0   0     198.51.100.3
172.20.15.240/28         qsfp0   0     198.51.100.3
172.20.16.0/23           qsfp0   0     198.51.100.3
172.20.18.0/23           qsfp0   0     198.51.100.3
172.20.24.0/24           qsfp0   0     198.51.100.3
172.20.25.0/24           qsfp0   0     198.51.100.3
172.20.26.0/24           qsfp0   0     198.51.100.3
172.20.27.0/24           qsfp0   0     198.51.100.3
172.20.28.0/24           qsfp0   0     198.51.100.3
172.20.29.0/24           qsfp0   0     198.51.100.3
172.20.30.0/24           qsfp0   0     198.51.100.3
172.20.31.0/24           qsfp0   0     198.51.100.3
172.21.0.0/16            qsfp0   0     198.51.100.3
172.30.0.0/16            qsfp0   0     198.51.100.3
172.31.0.0/16            qsfp0   0     198.51.100.3